1. Field of the Invention
This invention relates to an apparatus and process for frost control for space conditioning wherein ambient air is passed across a heat exchanger functioning as an evaporator such as in heat pump heating systems, freezers and refrigerator-freezers. More specifically, this invention is particularly applicable to heat pump systems for residential and commercial buildings comprising a compressor, a heat exchanger mounted within the interior of the building being conditioned, and an outdoor heat exchanger subjected to ambient air flow. The heat pump system normally includes a four-way valve for reversing the flow of refrigerant. During the cooling mode of the heat pump system, the indoor heat exchanger is the evaporator for the system, and the outdoor heat exchanger serves as the condenser. During the heating mode, these two heat exchangers trade functions; the indoor heat exchanger becomes the condenser rejecting heat to the interior of the building, while the outdoor heat exchanger becomes the evaporator picking up heat from the ambient air passing through the outdoor coils. More specifically, this invention relates to an improved ambient air heat exchanger wherein the coils of the evaporator heat exchanger are contained in a fluidized bed to enhance heat transfer and to diminish or totally eliminate frost formation on the evaporator coils during absorption of heat. The ambient air heat exchanger of this invention may be an outdoor heat exchanger functioning as an evaporator in the heating mode of a heat pump system, or a heat exchanger functioning as an evaporator in a freezer or a refrigerator-freezer system.
When the ambient air heat exchanger functions as an evaporator, particularly at ambient temperatures near freezing, there is a tendency for the moisture within the ambient air stream to condense and freeze on the evaporator surface which is at or below freezing temperature. Prior art solutions to this problem have focused on various methods to periodically defrost the evaporator coils. However, such systems are quite energy inefficient. This invention provides an energy efficient fluidized bed heat exchanger apparatus and system which transfers heat more efficiently and operates frost-free at near freezing ambient temperatures.
2. DESCRIPTION OF THE PRIOR ART
There have been many prior attempts to control frost accumulation particularly on the outdoor heat exchanger of a heat pump operating in the heating mode. One method common in small residential size heat pumps comprises a momentary mode reversal of the heat pump itself, wherein the flow of refrigerant is reversed changing the outdoor heat exchanger from its evaporator function to a condenser function. Defrost of the outdoor heat exchanger is accomplished by the condensation of hot vapor refrigerant in the outdoor heat exchanger. This method is applied by means of several embodiments differing mainly by the defrost control employed and the components utilized. For example, U.S. Pat. No. 4,007,603 teaches the use of a differential pressure switch across the outdoor evaporator to initiate and terminate the defrost cycle; U.S. Pat. No. 4,024,722 teaches defrost control by monitoring the surface temperatures of selected refrigeration components as well as the ambient atmospheric temperature; and U.S. Pat. No. 4,104,888 teaches defrost control by monitoring an operational parameter of the compressor sensitive to frost accumulation, such as compressor current. U.S. Pat. No. 3,024,620 teaches an outdoor heat exchanger configuration that results in decreased defrost time, while U.S. Pat. No. 3,240,028 teaches defrost time reduction by use of an auxiliary coil immersed in a hot oil bath which superheats the hot vapor refrigerant during defrost. U.S. Pat. No. 3,529,659 teaches the use of radiant heat from hot liquid refrigerant returning from the indoor heat exchanger to warm the air flow upstream to the main outdoor heat exchanger; U.S. Pat. No. 4,171,622 teaches the use of a tandem auxiliary outdoor heat exchanger which acts as a defroster during heating operations and a subcooler during cooling operations; and U.S. Pat. No. 4,178,767 teaches automatic fan motor reversal to blow air downward over the evaporator fins to assist gravity in removing water during the defrost cycle to prevent refreezing of condensate following defrost.
Other embodiments comprise use of bypass valves to reduce the defrost cycle time. For example, U.S. Pat. Nos. 3,274,793 and 3,041,845 teach the use of bypass valves to partly bypass the refrigerant metering device to permit a more rapid loading and heating of the outdoor heat exchanger during the first part of the defrost cycle. U.S. Pat. No. 3,068,661 teaches an increase in the operating temperature of the outdoor heat exchanger during defrost by partly bypassing the hot vapor refrigerant around the outdoor coil, thereby increasing the operating pressure of the indoor heat exchanger; and U.S. Pat. No. 4,158,950 teaches the use of bypass valves upon compressor shutdown to allow a free flow of hot vapor refrigerant into the outdoor heat exchanger until the system temperature equalizes.
Evaporator defrosting by refrigerant flow reversal is both energy inefficient and damaging to equipment. For marginally designed heat pump units, the energy consumption for frost control can amount to as much as 10 percent of the seasonable energy consumption.
Another method comprises the use of direct heat to the evaporator coil. For example, U.S. Pat. No. 3,918,268 teaches the use of direct heating means comprising an electrical resistance heater in thermal contact with the fins of the outdoor heat exchanger such that heat is transferred by conduction. Although simpler and less harmful to the compressor, electrical resistance heat defrosting is characterized by slow response, increased energy inefficiency, and is maintenance prone.
Embodiments comprising heat removal from fluidized beds have focused on the high temperature heat transfer from chemical reaction systems. British Pat. No. 587,774 teaches a method for controlling the reaction temperature in a system wherein the reaction zone is in indirect contact with the fluidized bed. U.S. Pat. No. 4,158,036 teaches the use of a secondary fluidized bed to remove heat from the effluent of an upstream high-temperature fluidized reaction bed. U.S. Pat. No. 4,096,909 teaches a fluidized bed process heater structure wherein the coils are mounted horizontally and are supported by the vessel walls.